Python: How to display per render layer properties in a UI list?
SceneRenderLayer cannot be assigned a bpy.props property.

From the docs on property defininitions


  Assigning to Existing Classes
  
  Custom properties can be added to any subclass of an ID, Bone and
  PoseBone.


Run a little check in console using builtin issubclass (don't need to consider bone or pose bone in this case)

# bpy.types.Scene.prop = ...
&gt;&gt;&gt; issubclass(bpy.types.Scene, bpy.types.ID)
True

# bpy.types.SceneRenderLayer.prop = ...
&gt;&gt; issubclass(bpy.types.SceneRenderLayer, bpy.types.ID)
False